It is known that individual portions of fish can be vacuum-packed in sachets ready for cooking. The sachets are then raised to high temperature, thereby cooking and pasteurizing them. This serves firstly to coagulate the proteins and secondly to increase shelf life.
Unfortunately, traditional cooking partially destroys the foodstuff. Firstly aqueous liquid containing coagulated proteins is exuded. Secondly, cooking deteriorates the Theological properties of the foodstuff. This deterioration is particularly marked with fish, where the flesh becomes firmer and dries out.
This causes deterioration in the organoleptic properties of the cooked foodstuff. In addition, fish immersed in an exuded aqueous liquid is unattractive in appearance and that can cause customers to reject it, particularly when the fish is in the form of individual portions vacuum-packed in transparent sachets and sold in self-service stores.
U.S. Pat. No. 2,705,868 proposes a method of cooking fish flesh that has been partially dehydrated and vacuum-packed under a pressure of several bars.
Although that method generally gives satisfaction, it gives rise to a loss of mass due to water loss and thus to a reduction in the sale value of the foodstuff.
It is known to add additives such as sugar polyphosphates and/or hydrocolloids to fish flesh, in particular prior to freezing, in order to avoid water loss by dripping during thawing. In such applications, the additives perform a cryoprotection function. The same additives are used in formulations for fancy products based on the flesh or pulp of fish, in which case they act as a binder; they enable fish debris or crumbs to be given shape and/or to be recompacted.
It is known to mix (75% to 90%) fish lumps and to provide cohesion between the lumps by coating them with a paste made up of fish flesh together with possible additives such as salts and/or proteins.
Patent FR-A-2 729 830 describes a method of preparing frozen raw fish that includes agglomerating large lumps of fish flesh. That method sets out to perform treatment at a temperature lying in the range -10.degree. C. to -2.degree. C., so as to keep the water constituting the fish flesh in a crystallized state and therefore avoid said water being exuded and lost. Nevertheless, when the foodstuff described in FR-A-2 729 830 is subsequently cooked, that will normally lead to water and protein being exuded and lost.
Hydrocolloids, and in particular carrageenins are conventionally used to reinforce the protein lattice, thus making it possible to obtain products that can be sliced. Nevertheless, the applications concerned are products in which the fish flesh is ground up (e.g. fish paste) and does not conserve its structure (Jensen, Food Marketing & Technology, August 1993, pp. 6-8).
It is known to treat fish with a solution of polyphosphate (soaking or steaming); polyphosphates appear to modify the surface state of proteins, thereby making it possible to avoid water loss from inside muscle, particularly when deep freezing fish (Gordon, Food Manufacture, July 1971, pp. 57-58).
The Applicant has found a particular way of incorporating water-retaining agents making it possible to conserve the fiber structure of fish flesh, and to limit to a considerable extent the amount of liquid that is exuded during cooking.
Without taking appropriate precautions, any attempt at causing water-retaining compounds to penetrate into the mass of the flesh, in particular by mixing under strong mechanical action, ends up with irreversible spoiling of the fiber structure due to physical breakage. Such spoiling consists in the lumps of flesh being broken up excessively to small dimensions that do not give the desired organoleptic qualities to the foodstuff and/or to spoiling of the fibers making up each lump.